Skin dryness is a common affliction of mankind which has attracted a great deal of attention in the soap and cosmetic industries. Many approaches have been taken to combat this condition; for example, emollients have been applied which cause a physical soothing effect on the skin, but which provide only temporary relief. Other materials referred to as humectants have been applied which cause a transfer of water from deep in the skin or from the air to the upper layers of the skin. Still other materials have been applied which occlude the loss of water from the skin surface, and yet other materials which affect skin metabolism have been used. The most popular approach has been to use emollients which alleviate the symptoms of dryness by filling in the rough areas and indentations of the outer skin layer (the stratum corneum) to provide a lubricating effect; however, as noted above, in most instances the relief is only a temporary surface phenomenon which does not affect the physiological functions of the skin.
It is generally recognized that natural skin oils and water provide the moisturizing effect in skin; therefore, the state of hydration of the stratum corneum is of major importance. Several factors which influence the state of hydration have been suggested, including the rate at which water reaches the stratum corneum from lower layers of the skin, the ability of the stratum corneum to hold water, and the rate at which moisture evaporates from the skin. These factors all concern "transpirational" water; that is, water which migrates to the stratum corneum from the lower skin layers. Soaps and cosmetics can affect skin moisture. Consequently, measuring the transpirational water loss (TWL) from the stratum corneum is one way to evaluate the moisturizing ability of various soaps and cosmetic products.
The present invention concerns toilet soap which tends to reduce moisture loss from the skin and improve skin feel. Toilet soap is comprised primarily of water soluble ammonium, alkali metal or alkanolamine salts of various fatty acids having chiefly from 12 to 18 carbon atoms. Typical examples of such soap bases are lauric, oleic, stearic and palmitic acids which may be derived from various sources, including animal fat, vegetable fat, vegetable oil, fish oil and whale oil. The sodium and potassium salts of tallow and coco fats are preferred, with sodium tallow/sodium coco soaps in the proportions of generally 90-10 to 50--50 being especially desirable. Particularly preferred proportions of these two soap bases are 85/15 and 70/30.
Ordinary toilet soap, when applied to the skin, can cause irritation and dryness. To overcome these symptoms, various additives have been incorporated into soap bars. For example, certain lower molecular weight polyethylene glycols, when incorporated into creams and lotions, have been reported to promote improved skin feel. In U.S. Pat. No. 2,309,722, Wilkes et al. disclosed the use of polyethylene glycols having molecular weights of from about 400 to 4000 in creams, lotions and other toilet preparations to effect improved softening and conditioning of the skin. We have found that when polyethylene glycols having subtantially higher molecular weights (e.g., from 88,000 to ca. 4,000,000) are incorporated into soap bars, superior skin feel will initially result when the bars are used. However, this attribute is illusory for these materials also tend to cause an increased loss of transpirational water. As a result bars containing these materials eventually tend to induce skin dryness.